Due to the increased role of statistics in current curriculum standards, there is at present a call for attention to secondary teachers' knowledge for teaching statistics in teacher education. This article reports the results of a study that answered this call by addressing mathematics teachers' knowledge for teaching graphing of bivariate categorical data. Novel curriculum materials to develop such knowledge were written then implemented with mathematics teachers in courses at four post-secondary schools. Results showed that prior to use of the materials, teachers' graphical competence was limited with teachers often utilizing frequencies when analyzing the data. Following use of the materials, they were more likely to correctly use relative frequencies in their analysis and expanded their knowledge of graphs to include segmented bar graphs. They also improved in their analysis of a student's graph and proposed response to the student, with more teachers noting the inappropriate use of frequencies and lack of label in the student-made graph post-instruction. Implications for development of teachers' knowledge for teaching statistics are discussed.